There is scant literature on the effectiveness of using transcranial direct current stimulation (tDCS) as an intervention modality for managing tinnitus. The current case series reflects the use of tDCS as an effective intervention for tinnitus while inhibiting the dominant temporoparietal cortex and simultaneous stimulating the non-dominant dorsolateral prefrontal cortex.

Transcranial direct current stimulation (tDCS) is a noninvasive focal neurostimulation technique that involves the application of a low-intensity electric current utilizing surface electrodes for modulating the underlying brain activity.[1] It is suggested that due to the shifts in the resting membrane potential (depolarization and hyperpolarization), cortical excitability is increased in the vicinity of the anode and reduced near the cathode.[2] It is also proposed that the prolonged effects of tDCS can be explained by the modifications of N-methyl-d-aspartate receptor efficacy, changes in gamma-aminobutyric acid activity, and modulation of interneurons, resulting in prolonged synaptic efficacy changes.[3]

There is a recent boost in the use of tDCS for the management of various neuropsychiatric and related conditions. Few studies have reported the effectiveness of tDCS for tinnitus in the Western literature, some with contradictory findings.[4],[5],[6] The current case series evaluates the effectiveness of this new intervention modality for managing tinnitus.

Case Presentation

Seven patients having a history of tinnitus were referred to the Department of Psychiatry for intervention after failing to respond to any medications by Department of Otorinolaryngology, All India Institute of Medical Sciences, New Delhi, India. All the participants were right handed. Details regarding the case histories of the participants are provided in [Table 1].

Only participant T1 had received a mechanical intervention in the past (3 months prior to receiving tDCS) in the form of a repetitive transcranial magnetic stimulation (rTMS) for 20 days and had improvement in tinnitus persisting during the course of rTMS but had re-emergence of symptoms after discontinuing rTMS over 5–6 days.

Intervention

Fixed parameters for tDCS (current intensity 2 mA, duration 20 min, slope 1 min, monochannel) and location [cathode at left temporoparietal area (LTA) and anode at dorsolateral prefrontal cortex (DLPFC) or occipital cortex] using 5 cm × 5 cm tDCS for tinnitus reduction were applied. Electrode localization was performed following the 10–20 international system. All participants received continuous 20 sessions over a 10-day period receiving two sessions per day with an interval of 3 h. Assessments were performed using the tinnitus handicap inventory.[7]

Results

As shown in [Table 2], there was a symptomatic improvement in all participants (P = 0.026). It was observed that the response rate decreased with increasing duration of illness. Those with less than 1 year of illness duration had better outcome in comparison to those with longer duration of illness. Best response was seen with the patient having less than 2 months of illness. There was no side effect reported by any participant. One participant did not complete all the sessions citing personal reasons unrelated to tDCS.

The least improvement was observed in two participants having comorbid moderate depression. One more patient having mixed anxiety and depression (milder form of depression), however, showed a better response. It was also observed that a family history of tinnitus was associated with better response even with a long duration of illness. The presence of multiple comorbidities did not prevent a good response to tDCS.

Discussion

There was significant symptomatic relief in patients having tinnitus using tDCS as a neuromodulatory approach. Cathodal application at LTA and anodal at DLPFC was found to be beneficial for participants.

Western literature has recently reported the variable effectiveness of tDCS in tinnitus in randomized controlled trial (RCT) with studies reporting negative[4],[5] and positive outcomes.[6] Forogh et al. used anodal tDCS over left LTA (stimulating the LTA) while current study provided cathodal tDCS over left LTA (inhibiting the LTA).[4] There are other studies reporting positive outcomes with cathodal tDCS to LTA[8] while others have demonstrated better outcomes with anodal tDCS.[9],[10] This differential response to anodal and cathodal application to same cortical area may be attributed to a disrupting effect of ongoing neural hyperactivity, independent of the inhibitory or excitatory effects. There might be a role of functionally connected cortical structures in modulating the response indirectly. Neurophysiological hypothesis of tinnitus is still nonconsensual but proposes that tinnitus is related to either auditory deafferentation or a deficit in noise canceling or a combination of both.[11] It might be possible that not only the cortical area but also the duration and intensity of applied tDCS current may filter the response to different electrode.

Another RCT utilized only five sessions over five consecutive days.[5] In the current study, it was found that effective outcome is initiated mostly after five sessions. The negative findings of previous studies could be attributable to understimulation/inhibition of the intended cortical areas. Other researchers utilizing higher intensity (2 mA) and longer duration (20 min) of stimulation have also found more effective outcomes.[6]

In addition, it was observed that there was a differential response to tDCS in that individuals with a shorter duration of illness responded more favorably than others.

Although the generalization of the findings is limited due to being an observational study with small sample without comparison data, the study strengthens the existing literature in using tDCS as an effective intervention for tinnitus. Researchers should direct their attention toward early initiation and intense as well as longer duration protocols of tDCS intervention for participants with tinnitus.